1. Field of the Invention
The present invention relates to etch techniques used in integrated circuit manufacturing and, in particular, to a method of reducing stop layer loss in photoresist stripping subsequent to oxide etching by introducing a scavenger for fluorine, such as carbon monoxide, into the resist strip etch chemistry.
2. Description of the Related Art
The direction of semiconductor integrated circuit manufacturing technology is toward continuously improving the density of integrated circuit devices. One of the important goals in achieving increased device density is raising the selectivity of the etch step used to strip a photoresist mask following use of the mask in a patterning step.
More specifically, after a photoresist layer has been formed and defined on an underlying layer to be etched, such as an oxide layer, patterning of the oxide layer generally includes two process steps. First, referring to FIG. 1, the oxide, using overlying defined and developed photoresist (PR), is subjected to oxide etching 100. Second, a photoresist stripping step 200 is performed to remove the photoresist to finish the patterning process.
In some cases, as with the Applied Materials HDP 5300 etcher, the two steps are performed sequentially in the same process chamber without transferring the wafer between chambers. In other cases, as shown in FIG. 2, a transition process 150 may be employed between the two steps to change and stabilize gas conditions in the chamber.
The manufacture of semiconductor integrated circuit devices often includes the formation of oxide layers for purposes of insulation and device formation. Accuracy in defining the pattern of the oxide layer is a vital factor in the design of the integrated circuit.
Numerous developments in etching the oxide layer with high selectivity to an underlying stop layer have been made. One widely utilized technology for oxide etching is the use of a fluorinated chemical etchant. During the oxide etch step, a fluorine-containing polymer is formed as a passivation material and functions as an etch stop to cover the underlying stop layer, such as silicon nitride or polysilicon, to increase the oxide etch selectivity dramatically.
For example, U.S. Pat. No. 5,286,344 to Blalock, et al. discloses a fluorinated chemical etchant system for etching an oxide layer on an underlying silicon nitride stop layer. The chemical etchant system disclosed by Blalock, et al. includes a fluorocarbon etchant material and an additive material such as CH2F2. The selectivity of the oxide etching is increased by the formation of polymer on the underlying nitride surface.
U.S. Pat. No. 5,423,945 to Marks, et al. discloses a method of etching an oxide over a nitride with high selectivity. In the Marks, et al. process, the oxide is plasma etched with a carbon and a fluorine-containing etch gas in the presence of a flourine scavenger, therby forming a carbon rich polymer which passivates the nitride. This polymer is inert to the plasma etch gases and, thus, provides high selectivity to the etch process.
FIG. 3 shows an example of a self-aligned contact (SAC) process in which an oxide 10 is etched using a patterning photoresist 12. As discussed above, due to the nature of the oxide etch chemistry, a fluorine-containing polymer 14 forms as an etch stop that passivates the silicon substrate 16 and a nitride spacer 18 from etching. Following oxide etching, the pattern-defining photoresist 12 and the fluorine-containing polymer 14 are then removed. During plasma stripping of the photoresist 12 and the fluorine-containing polymer 14, fluorine is released into the plasma from the dissociation of the polymer. The photoresist stripping plasma, now including free fluorine released from the polymer, etches the unprotected stop layer to the detriment of the device structure. More specifically, referring to FIG. 4, the stop layers, i.e., the substrate 16 and the nitride spacer 18, are etched by the fluorine-containing plasma during the removal of the polymer in the photoresist stripping step.
Thus, as the density of integrated circuits becomes greater, the tolerance for stop layer loss becomes smaller and the problem becomes more serious. Losing several hundred Angstroms in a stop layer, which could have been neglected in the past, has now become a crucial limit in integration circuit density. For densely packed devices like memory cells, it is difficult to compensate for stop layer loss by increasing deposition within the narrow spaces of the memory cell array and, thus, the loss of the stop layer damages the function of the device.
In general, a conventional oxide etching process involves the utilization of a fluorocarbon etch gas, such as CF4, C2F6, C3F8, CH2F4 and the CxFy group. The rich amount of free fluorine atoms and fluorine-containing radicals in plasmas resulting from these compounds forms the fluorine-rich passivation polymer described above. The polymer typically contains about thirty percent by weight of carbon and about sixty percent by weight of fluorine. Even with technologies that form a polymer that contains lower amounts of fluorine or a carbon-rich polymer, the polymer still contains about forty percent by weight of fluorine.
The present invention provides a method of reducing stop layer loss in a photoresist stripping plasma etch step by utilizing a scavenger for fluorine in the photoresist strip etch chemistry. In accordance with the invention, the scavenger reacts with the free fluorine generated by the dissociation of fluorine-containing polymer during the resist strip step and reduces the content of free fluorine in the plasma. By reducing the amount of fluorine available to attack the stop layer, the loss of the stop layer can be greatly reduced.
The method includes the steps of providing a substrate with a fluorine-containing polymer, a photoresist layer, a patterned layer, and at least one stop layer formed thereon. The fluorine-containing polymer is formed on the stop layer during an etching process and serves as an etch stop for achieving high selectivity. Following the etching process, the photoresist layer is stripped in a plasma derived from an oxygen-containing gas and a fluorine scavenger, with some content of free fluorine. The content of free fluorine is released from the fluorine-containing polymer during photoresist stripping. The fluorine scavenger reduces the concentration of free fluorine in the resist strip plasma.
The foregoing aspects of the present invention will become more readily appreciated and better understood by reference to the following detailed description which should be considered in conjunction with the accompanying drawings.